Fracturing relief method for stress concentration of remaining ore pillars in overlying goaf

Abstract

Provided is a fracturing relief method for stress concentration of remaining ore pillars in an overlying goaf, including first, performing directional fracturing on a roof to optimize the stress of the roof and reduce the source of force; secondly, performing pulse fracturing on a coal pillar to produce a crack network, weaken the stiffness of the coal pillar and reduce the bearing capacity of the coal pillar; and finally, performing pulse fracturing on a floor strata of the coal pillar to reduce the ability of transferring stress concentration thereof. A drilling machine is used for separately constructing fracturing drill holes in a roadway to a set depth at an interval in a direction oblique to the coal pillar in an upper goaf. The roof, the coal pillar and the floor can be fractured by an oblique fracturing hole in a sublevel retreating manner. The position of the directional fracturing of the roof is approximately 1 m above the middle of a main roof above the coal pillar. The method reduces the width of the lower coal pillar, improves the coal mining rate, reduces the deformation of the lower coal roadway, effectively solves the problems of mine pressure passing through the coal pillar on the working face of the lower coal seam, rock burst, and coal and gas outburst in the mining of the lower coal seam, and simultaneously has the advantages of high safety factor, simple method, convenient construction and low cost.

Description

BACKGROUNDTechnical Field[0001]The present invention relates to a fracturing relief method for stress concentration of remaining ore pillars in an overlying goaf, which belongs to the technical field of mining.Description of Related Art[0002]Long wall mining is adopted for most underground coal mines. The air return roadway or transportation roadway on the side of goaf is usually separated from the goaf by arranging coal pillars. On the one hand, the stability of the surrounding rock of the roadway on the side of the goaf is ensured; and on the other hand, the coal pillars can play a sealing role, thus isolating harmful gas in the goaf, preventing air leakage, spontaneous combustion of coal, etc. The width of the arranged coal pillars is generally determined according to the thickness, hardness and integrity of the coal seam.[0003]Room and pillar mining is adopted for most non-coal underground mines. In order to control the surrounding rock of the mined seam, a large number of ore p...

AI Technical Summary

Benefits of technology

The present invention provides a method for fracturing remaining coal pillars in an overlying goaf to improve coal mining efficiency, reduce deformation of the lower coal roadway, and solve problems of rock pressure and gas outburst. The method uses high-pressure and pulse hydraulic fracturing to create a network of cracks in the coal pillar, floor, and roof, which weakens the ability of the coal pillar to transfer stress concentration. The method is simple, safe, reliable, and cost-effective.

Problems solved by technology

When mining coal pillars or coal seams or mine bodies under pillars, stress concentration will occur, which will cause the problems of large deformation of the lower roadway and rock pressure passing through the coal pillar on the working face of the lower coal seam, and the problems of abnormal pressure or support crushing on the working face, rock burst, and coal and gas outburst in the mining of the lower coal seam.

However, there are the following problems when the method of enlarging the coal pillar is adopted: (a) two-seam coal: the loss of the coal pillar in the lower seam is increased, the coal mining rate is reduced and the resources are wasted; and (b) multi-seam coal: the method of enlarging the coal pillar is adopted for multi-seam mining, the coal pillar is caused to be larger and larger, and a lot of resources are wasted; once the left width of the coal pillar is not favorable, it will also cause pillar instability and bring a large hidden danger to safety production.

The use of the combined support technology of bolts, cables, steel sheds and timber pillars will largely increase the cost of roadway support.

However, the measures of strengthening roof management and on-site management have not really relieved the problem of stress concentration of remaining coal pillars in the overlying goaf, and it is still prone to safety accidents due to mining.

However, when the method of coal seam water infusion is adopted, the effect on solving the problem of rock burst is not remarkable, and the time of water infusion is longer.

When large-diameter pressure relief drilling is adopted, dense holes are usually arranged, the number of the holes is large and the labor cost is large; the mining of the protective seam is restricted by the distance between coal seams.

a. the traditional safety management of blasting to cave the roof is complex: it involves the management and transportation of explosives and detonators, and the blasting should strictly implement some systems such as “blasting once checking gas three times” and “three-man linkage blasting”;

b. there are potential safety hazards: practice shows that a large number of harmful gases such as CO produced instantaneously by large-scale blasting have a tremendous impact on mine ventilation safety management; for high-gas underground mines, it is not suitable to adopt blasting to break the coal pillar because of the potential danger of gas explosion induced by blasting sparks; and

c. the economic cost of blasting is high: when the coal pillar is broken, the spacing between blast holes is usually very small, and therefore a large number of explosives and initiating explosive devices such as detonators are needed.

However, the stress cannot be removed and can only be transferred.

For the coal pillar itself, the main purpose is to produce as many dense cracks as possible to break the coal pillar and reduce the stiffness of the coal pillar, thus reducing the bearing capacity of the coal pillar.

By performing hydraulic fracturing, the floor strata of the coal pillar are weakened, thus weakening the ability to transfer stress concentration.

When the pulsating pressure wave propagates to the interface between water and coal at the crack tip, it produces pulsating incident waves and pulsating reflected waves, and the reflection, superposition and reciprocation of the pulsating pressure waves cause the phenomena of the amplitude expansion of the pulsating pressure waves and increase of pressure; because of the effect of friction resistance, the phenomenon of pressure increase is also caused at the crack tip; and since the pulsating pressure will cause fatigue damage to the coal body, the pulsating pressure waves can produce more cracks under the effect of small pulsating pressure.

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